A case of acute intermittent porphyria associated with SIADH is presented. A 26-year-old woman was admitted to Inje University Paik Hospital because of severe abdominal cramp pain. Pain attacks occurred 10 months before entry, so she underwent an explolaparotomy under the impression of acute appedicitis, which was not revealed. On admission, she showed variable neurologic symptoms such as seizure, confusion, disorientation and transient optic nerve dysfunction. Acute intermittent porphyria was diagnosed by the Watson-Schwartz test and a quantitative assay of porphobilinogen and δ-ALA on 24 hrs. collected urine. It was induced by an upper respiratory tract infection or less probably, medications which were associated with SIADH. SIADH was diagnosed with hyponatremia, considerable urinary Na loss in spite of hyponatremia and a higher urine osmolality than that of plasma. The patient was treated with hypertonic saline infusion, fluid restriction, chlorpromazine and meperidine. Upon discharge, she was improved.